Additive for chromium electrolytes

ABSTRACT

The fluorine surfactant-free, long-term-stable and biodegradable additive for chromium electrolytes lowers the surface tension and hence improves the chromium deposition process, especially in the application for the electrolytical chromium-plating. Preferred additives are CH 3 (CH 2 ) n SO 3 H and the salts thereof with n from 10 to 18. These additives are also useful as component in polymer mordants as pretreatments for polymer metallizations.

The present invention relates to the field of additives for chromiumelectrolytes, especially to the field of the additive surfactants forchromium electrolytes, and to the field of the additives for chromicacid solutions applied for polymer metallizations.

For electrochemical chromium-plating, which typically takes place inchromium electrolytes at high concentration of aggressive chromic acid,a wide variety of different additives are proposed in order to preventthe formation of aggressive spray mists being caused by the formation ofhydrogen under the electrolysing conditions. However, it has been foundto be difficult in practice to find compounds (additives) which, on theone hand, have the intended favourable properties but, on the otherhand, survive the aggressive conditions in the chromium-platingunchanged or at least over a longer time periode.

Particularly suitable for the reduction of spray mist are foam-formingwetting agents (surfactants) which, by lowering the surface tension, notonly reduce the spray losses but also greatly reduce the entrainment ofthe chromium electrolyte. For this purpose, for exampleperfluoroalkylsulphonic acids (PFOAs) have been proposed. These productsare also stable to the highly oxidative properties of chromic acid.However, their use is—especially in view of the environment—problematicand already banned in many applications. These fluorine surfactants arenot biodegradable, since they do not undergo any photolytic, hydrolytic,oxidative or reductive transformation whatsoever. They are biodegradedneither aerobically nor anaerobically. Owing to their physicochemicalproperties, perfluoroalkylsulphonic acids remain as end metabolites andare not degraded any further.

DE-B 1 034 945 has proposed already up to 10 g/l alkylmethylsulphonates(i.e. salts of bis or monoalkylmethan sulfonic acid) as additives, whichare said to have the properties of a surfactant and simultaneously bringabout process-improving influences with regard to the smoothing of thechromium layer. However, these additives are unsuitable in practice,since they decompose in the course of chromium-plating within a veryshort time.

From DE-C 37 23 198 up to 10 g/l of perfluoralkyl groups containingesters of phosphoric acid are known as additives, which are additionallyadded to a chromium electrolyte, having already a content of 125 mg/ltetraethylammonium perfluorooctyl sulfonate besides 250 g/l CrO₃ and 2,5g/l sulphate ions. It is said that no spray mist is present under asurface tension below 30 mN/m and a temperature of 55° C.

DE-C 39 33 896 describes the addition of saturated aliphatic sulfonicacids or their salts or their halogen derivatives having maximum 2 Catoms and maximum 6 sulfonic acid groups during the electrolyticchromium-plating. The addition of fluoro compounds are also mentioned.The working current density ranges from 10 to 1200 A/dm². In specificexamples 10 mg/l ammonium fluorooctan sulfonate is applied as wettingagent based on 300 g/l CrO₃ and 1.3% (related to CrO₃) H₂SO₄, thecurrent density is 50 A/dm² and the temperature 55° C.; the additive isa saturated aliphatic sulfonic acid having 1 C and 1 sulfonic acid group(3.2 g/l).

The galvanic chromium electrolytes of DE-A 43 05 732 contains 0,4 to 10g 1,3-propandisulfonic acid-2-sulfinic acid and/or1,2,3-propantrisulfonic acid or the salts thereof besides 150 to 400 g/lCrO₃ and 1 to 4% by weight (related to CrO₃) H₂SO₄. In a specificexample 3 to 5 g additive are applied at 55° C. and a current density of50 A/dm².

The galvanic chromium electrolyte of DE-C 44 30 923 contains 2 to 10 g/lmethansulfonic acid and 2 to 20 g/l MgSiF₆ as additive to CrO₃. In aspecific example there are also 0,1 ml/l tetraethylammoniumperfluorooctyl sulfonate and 1 g/l sugar present. The chromiumdeposition is conducted at 40° C. and a current density of 1.82 A/dm²and during 1 h.

From DE-C 198 28 545 a chromium electrolyte is known containing besides100 to 600 g/l CrO₃ and sulphate ions in a relation of Cr⁶+to S0 ₄ ²⁻from 90 to 120 to 1 additionally 0.01 to 3.0 g/l Na-2-hydroxyethansulfonate or its free acid. At a current density of 30 to 90 A/dm² thechromium deposition is conducted in 3 steps from 44 to 57° C. duringmore than 1 h.

The chromium layers of De-A 102 55 853 are produced in an electrolytecontaining 50 to 600 g/l CrO₃, 0,5 to 10 g/l H₂SO₄, 1 to 20 g/l of analiphatic sulfonic acid having 1 to 6 C atoms and 10 to 200 g/l of amolybdate, vanadate or zirkonate. In the examples the electrolytecontains 250 g/l CrO₃, 2.5 g/l H₂SO₄, 4 g/l methan sulfonic acid and 100g/l (NH₄)₈Mo₇O₂₄·4 H₂O; the electrolyse is conducted at 55° C. and at acurrent density of 40 A/dm² during 30 min. or at 50 A/dm² during 120min.

The electrolyte of DE-B 10 2004 019 370 does not contain—compared withthe beforementioned solution—molybdates, vanadates or zirkonates. In theexamples methan sulfonic acid (as a solution of 70%) in an amount of 9ml/l at 60 to 70° C. during 30 min. at 30 to 80 A/dm² is applied. Thecurrent efficiency is at ≦12% (as in the beforementioned solution).

The chromium electrolytes known from the prior art are either onlysuitable for very specific applications, or show thedisadvantages—already discussed before—caused by a fluoro part in theadditive molecule.

It is therefore an object of the present invention to find an additivefor chromium electrolytes as well as for chromic acid solutions appliedfor polymer metallizations which reduces the disadvantages detailedabove.

This object is achieved by an additive for aqueous chromic acidsolutions, being free of fluorine surfactants and being biodegradableand having in a chromic acid solution which contains the said additive asurface tension of ≦35 mN/m, and having in the application in theelectrolytical chromium-plating, determined at 45° C. and 6000 Ah ofcharge passage, in the chromic acid solution which contains the saidadditive a stability of ≧4 h.

It should be noted that the term “additive” within the present inventioncan relate either to an individual substance or to a substance mixture;for reasons of readability and clarity, however, “additive” is onlyreferred to in the singular within the present invention. If theadditive used is a substance mixture, what is meant in general is thatthe substance mixture has the properties described, but the individualcomponents of the mixture may also only have the properties described.

In the context of the present invention, “stability” means especiallythe lasting efficacy of the additive with regard to the surface tensionunder the chemically demanding conditions of a chromium electrolyte inthe application in the electrolytical chromium-plating. In particular,in the context of the present invention, “stability” over a certainperiod means that the surface tension increases by not more than 5 mN/mover this time.

In particular, in the context of the present invention, “chromiumelectrolyte(s)” and/or “chromic acid solution(s)” are understood to meanchromium electrolytes or chromic acid solutions which—besides chromicacid and water—comprises catalysts and/or further acids among othercomponents.

The content of chromic acid in the solution and the electrolyte,respectively,—related to CrO₃—is normally between 120 and 450 g/l.

It has been found that, when such an additive according to the presentinvention is added to chromic acid solutions in most applications, butespecially in chromium-plating operations, at least one, usually morethan one, of the following advantages can be achieved:

-   -   The use of the inventive additive improves the operation of the        chromium electrolytes in a lasting manner.    -   The use of the additive leads to the formation of significantly        smaller gas bubbles, which is associated with a drastic        reduction in the emission nuisance.    -   It is likewise possible to considerably reduce entrainment        losses.    -   Depending on the process, when the additive is used, the        dispersibility of the electrolytes in many applications is        improved.    -   The additive does not adversely affect the properties of the        chromium layer, not even with regard to layer properties such as        hardness, crack network or structure.

A chromic acid solution, which contains e.g. 0.1 g/l of additive and 250g/l of chromic acid, preferably has a surface tension of ≦28 mN/m, evenmore preferably ≦25 mN.

A chromic acid solution, which contains e.g. 0.1 g/l of additive and 400g/l of chromic acid, preferably has a surface tension of ≦35 mN/m, evenmore preferably ≦30 mN/m.

An additive according to the present invention has at 45° C. in achromium electrolyte which contains 0.1 g/l and 270 g/l of chromic acidpreferably has a stability of ≧8 h, even more preferably of ≦12 h.

The additive is free of fluorine surfactants. This is understood to meanespecially that the additive does not contain any organofluorinecompound, or that the proportion of organofluorine compounds in theadditive is below the detection limit. Furthermore, the additive isbiodegradable. This is understood to mean especially that, according toOECD criteria, ≧99.5%, preferably ≧99.8%, of the additive has degradedin the screening test after 8 days. In many applications, such anadditive contributes to minimizing expenditure with regard to preventingthe contamination of the environment, or even to making it entirelysuperfluous.

A chromium electrolyte, which contains e.g. 0.1 g/l of additive and 250g/l of chromic acid, preferably has a current density of 30 A/dm² to 60A/dm², even more preferably 40 A/dm² to 50 A/dm².

A chromium electrolyte, which contains e.g. 0.2 g/l of additive and 350to 400 g/l of chromic acid, preferably has a current density of 5 A/dm²to 25 A/dm², even more preferably 8 A/dm² to 20 A/dm².

In a preferred embodiment of the present invention, the additivecomprises at least a surfactant selected from the group consisting oflong-chain alkylmonosulphonic acids, long-chain alkyldisulphonic acids,long-chain alkylpolysulphonic acids, salts of the long-chainalkylmonosulphonic acids, salts of the long-chain alkyldisulphonic acidsand salts of the long-chain alkylpolysulphonic acids.

In this context, “long-chain” is understood to mean at least 4 C atoms.The long-chain alkyl radicals are preferably unbranched, but it is alsopossible to use branched alkylmono-,-di- and -polysulphonic acids andsalts thereof. The salts used are alkali metal salts, alkaline earthmetal salts, NH₄ ⁺ salts, NR₄ ⁺ salts (where R═C₁ to C₄ alkyl).Polysulphonic acids have 3 to 6 sulfonic acid group per molecule.

In a preferred embodiment of the present invention, the additivecomprises, as surfactant at least one compound CH₃(CH₂)nSO₃H or saltsthereof, where n is 10 to 18. In practice, these compounds often have aparticularly elevated stability and are preferred in this respect. Morepreferably, the additive comprises such surfactants, where n is 12 to17; even more preferably n is 14 to 16.

The present invention also relates to use of the inventive additive as asmoothing agent in chromium electrolytes. Usually the concentration ofadditive is between 0.05 g/l and 20 g/l, preferably from 0.1 g/l to 10g/l, and more preferably from 1 g/l to 3 g/l.

The present invention also relates to use of the inventive additive asan additive in polymer mordants. It has been found that, surprisingly,the inventive additive can be used not only in chromium electrolytes butalso in this pretreatment of polymer metallizations. In these mordants,the additive has a wetting effect and lowers the surface tension of thechromic acid-containing mordants. The positive influence on chromic acidmist formation and entrainment is comparable to the effects describedabove for chromium electrolytes. The amounts of surfactant in a chromicacid-containing mordant are basically identical with its amounts inchromium electrolytes.

The aforementioned components to be used in accordance with theinvention, and those described in the working examples, are not subjectto any particular exceptional conditions in their size,three-dimensional configuration, material selection and technicaldesign, such that the selection criteria known in the field of use canbe used without restriction.

Further details, features and advantages of the subject-matter of theinvention are evident from the description of an inventive example.

EXAMPLE

In a bath containing 400 g/l of chromic acid, 5 g/l of phosphoric acid,3 g/l of potassium nitrate, 3 g/l of rare earth fluorides (e.g. cerium,lanthanum) and, as the inventive additive, 2 g/l of sodiumpentadecanesulphonate (i.e. in the general formula is n=14), at atemperature of 20 to 25° C. and a current density of 20 A/dm², a blackchromium coating was performed.

It was possible to lower the surface tension to a value of 29.8 mN/m bythe addition of the inventive additive. The chromic acid solutioncontinued to be stable after an application time of 4 h during theelectrolytical chromium-plating, i.e. the surface tension is onlyincreased by much less than 5 mN/m.

In the subsequent studies, it was found that the chromium layerdeposited had a very uniform appearance. In particular, thedispersibility of the electrolyte was improved. The testing of severalsheets gave a dispersion of the chromium layer which was improved by anaverage of 1.0 to 1.5 cm according to corresponding tests in a Hullcell.

Materials and Testing Methods

The surface tension was measured with a K8 tensiometer from Krüss GmbHin Hamburg. The unit works by the Du Noüy ring method. The force of aliquid lamella drawn up by the ring is measured. The liquid is raiseduntil there is contact of the ring with the surface. With the aid of atorsion balance, the force required to raise the platinum ring ismeasured. The further the ring is pulled out of the liquid, the greaterthis is. At the point of the highest force applied, when the liquidlamella breaks off, there is a force equilibrium from which the surfacetension of the liquid can be calculated. The ring geometry is taken intoaccount by means of an instrument-specific calibration by themanufacturer.

The current density is determined by means of a measurement of thecurrents with an amperemeter and reference to the known surface geometryof the components to be chromium-plated.

The dispersibility of an electrolyte is determined by the evaluation ofsheets after tests with the electrolyte in a Hull cell. The dispersionof the chromium layer is determined by measuring the expansion of thecoated surface on the sheet after it has passed through the test run.The measurement is effected with a ruler. In general, several sheets arecoated and measured under the same conditions in order to obtainreliable averages.

1-9. (canceled)
 10. Additive for chromic acid solutions, being free offluorine surfactants and biodegradable and having in a chromic acidsolution which contains the said additive a surface tension of ≦30 mN/m,and having in the application in the electrolytical chromium-plating,determined at 45° C. and 6000 Ah of charge passage, in the chromic acidsolution which contains the said additive a stability of ≦4 h. 11.Additive according to claim 1, comprised in a chromic acid solution andused in the electrolytical chromium-plating, this chromic acid solutionhas a current density of 35 A/dm² to 60 A/dm².
 12. Additive according toclaim 1, comprising at least one surfactant from the group consisting oflong-chain alkylmonosulphonic acids, long-chain alkyldisulphonic acids,long-chain alkylpolysulphonic acids, salts of the long-chainalkylmonosulphonic acids, salts of the long-chain alkyldisulphonic acidsand salts of the long-chain alkylpolysulphonic acids.
 13. Additiveaccording to claim 1, comprising at least one surfactant beingCH₃(CH₂)nSO₃H or salts thereof, where n is 10 to
 18. 14. Method of useof an additive according to claim 1 as a smoothing agent in chromiumelectrolytes.
 15. Method of use according to claim 5, wherein theconcentration of the additive is between 0.05 g/l and 20 g/l.
 16. Methodof use of an additive according to claim 1 as an additive in polymermordants as pretreatments for polymer metallizations.
 17. Method of useof an additive according to claim 3 as a smoothing agent in chromiumelectrolytes.